Parasitic oscillation problem untraceable

[Bieworm]

I can't seem to zoom in on the PI input grid wave...

20210107_224833.jpg

[Bieworm]

Now I'm going to bed... good night

[JMPGuitars]

mA isn't amplitude. I'm pretty sure that was a typo.

LOL, I dunno what you're talking about, it says mV on my post.

[JMPGuitars]

Now I'm going to bed... good night

And when you wake up, make the amp match my circled changes:

Preamp.jpg

PowerAmp.jpg

Note the tube changes I listed on the attached voltage chart. Take new voltages after making the changes.

Thanks,
Josh

[crgfrench]

Just curious, what does swapping the triode pin connections in V1 & V2 accomplish?

[Bieworm]

Thanks Josh!!

[Bieworm]

Just curious, what does swapping the triode pin connections in V1 & V2 accomplish?

It's layout related to avoid wires crossing.

[TriodeLuvr]

Just curious, what does swapping the triode pin connections in V1 & V2 accomplish?

It's layout related to avoid wires crossing.

That can make a big difference. I saw it in the prototype of my current project when I was testing with octals. At one point, I tried to cross the signal wires over each other for convenience, and there was a loud complaint. OTOH, I don't see any indication of oscillation in the scope traces you posted. Very strange. Could this be a frequency response problem? Have you swept it with your generator to confirm that it's doing what it should?

Jack

[Bieworm]

Just curious, what does swapping the triode pin connections in V1 & V2 accomplish?

It's layout related to avoid wires crossing.

That can make a big difference. I saw it in the prototype of my current project when I was testing with octals. At one point, I tried to cross the signal wires over each other for convenience, and there was a loud complaint. OTOH, I don't see any indication of oscillation in the scope traces you posted. Very strange. Could this be a frequency response problem? Have you swept it with your generator to confirm that it's doing what it should?

Jack

I don't know. I haven't ran it through the speaker. Just took some voltages yesterday for Josh. And further on I started scoping. More out of curiosity what the signal looks like and what values they represent. I'm just barely getting a hold of the scope-thang. Never did that before with electronic devices. I only used a scope back in the nineties for measurements of compression and rebound af shock absorbers (Tenneco Engineering)
I wonder howcome I can't get a decent wave size when measuring the PI....
Next week I have a better scope, so I'll look forward to measuring with better equipment.

[crgfrench]

I wonder howcome I can't get a decent wave size when measuring the PI....

Did you try adjusting the "Scale" knob (the larger white dial just above the probe jack)?

[Bieworm]

I wonder howcome I can't get a decent wave size when measuring the PI....

Did you try adjusting the "Scale" knob (the larger white dial just above the probe jack)?

I'm doing nothing but that. With the other tubes there's no problem of scaling in. But with the PI it's not the case... I'm puzzled.
Next week I'll have my Siglent 1102 CML and will be evaluating again with better equipment. If I don't understand something I'll ask you directly, since you have one of those

[TriodeLuvr]

I'm doing nothing but that. With the other tubes there's no problem of scaling in. But with the PI it's not the case... I'm puzzled.

I wonder if this indicates a gain problem directly ahead of the PI? If the earlier stages are having to compensate for a loss later in the chain, they might be overloading too early. On the other hand, I think you're seeing roughly the correct peak voltage on the grids of the output tubes (assuming I'm interpreting your scope traces correctly). It's a mystery...

Jack

[Bieworm]

Now I'm going to bed... good night

And when you wake up, make the amp match my circled changes:
Preamp.jpg

PowerAmp.jpg


Note the tube changes I listed on the attached voltage chart. Take new voltages after making the changes.

Thanks,
Josh

Here the voltages after the change:
Still fizz...

20210108_204247.jpg

[JMPGuitars]

Still fizz...

Those voltages are with a 12AT7 in the PI?

There's a few things left to try.

First, try lowering the plate resistors on the PI and bring the plate voltages up. Let's try for 200 to 225 and see what (if any) effect that has.

Second, make the snubber circuit.

Besides that, you need to get another set of power tubes (or two sets) to rule them out.

Maybe also a good time to do a highlighter test again.

Another option would be an NFB.

[Bieworm]

Still fizz...

Those voltages are with a 12AT7 in the PI?

There's a few things left to try.

First, try lowering the plate resistors on the PI and bring the plate voltages up. Let's try for 200 to 225 and see what (if any) effect that has.

Second, make the snubber circuit.

Besides that, you need to get another set of power tubes (or two sets) to rule them out.

Maybe also a good time to do a highlighter test again.

Another option would be an NFB.

No a 12ax7
I know I should get other tubes. The ones in there are new ehx 6v6gt
Maybe the OT is faulty.. that would be a bummer.
This is a tricky thing to find ...
NFB? Isn't that bad for nice overdrive?

[JMPGuitars]

No a 12ax7
I know I should get other tubes. The ones in there are new ehx 6v6gt
Maybe the OT is faulty.. that would be a bummer.
This is a tricky thing to find ...
NFB? Isn't that bad for nice overdrive?

I said to switch to 12AT7 for a reason. Try it out and see what happens. If not better, raise the plate voltages.

NFB is kinda bad for the distortion we prefer, but it's good for some issues.

[Bieworm]

I said to switch to 12AT7 for a reason. Try it out and see what happens. If not better, raise the plate voltages.

NFB is kinda bad for the distortion we prefer, but it's good for some issues.

Ok here is with the PI 12at7
Plates 147V
Grids 47V
Cathodes 73V

PI 12at7 with 56k (input) +68k load resistor ( that's what I have left in the box
Plate (input) 175V
2nd plate 171V
Grids 53V and 57V
Cathodes 85V both

NFB tested with 100k and then 820R

Same fizz

Still want to lower the plate resistors more?

10k:
Plates 230V
Grids 84V
Cathodes 112V

Same fizz

[geoff 1965]

have you tried a 47 or 100pf snubber across the phase plates?

[geoff 1965]

have you tried a 47 or 100pf snubber across the phase plates?

[Bieworm]

have you tried a 47 or 100pf snubber across the phase plates?

There is already 100pf there